Switching power supply

ABSTRACT

A power supply uses a switching element of a desired voltage withstanding property to obtain a desired conversion output. The power supply includes a converter transformer having primary and secondary windings loosely coupled to one another, and an auxiliary winding with a predetermined number of turns wound contiguously from one end of the primary winding to an end portion of the converter transformer. An input voltage smoothing capacitor is connected on one side to an end of the primary winding of the converter transformer through a forwardly directed diode. A switching element is connected to the other end of the primary winding. A control circuit controls the switching element to perform switching so that an output voltage of the secondary winding has a predetermined voltage value (or is within a predetermined voltage range). A second capacitor is coupled across opposite ends of the auxiliary winding via the forwardly directed diode, to boost-up voltage applied to the converter transformer.

TECHNICAL FIELD

The present invention relates to a switching power supply suitable foruse, for example, with a television receiver. More particularly, theinvention relates to a resonant-type of switching power supply which caneffect efficient power conversion with a simple configuration.

BACKGROUND ART

Switching power supplies for use with televisions, computer monitors,and the like, are typically characterized as “soft switching”, whichdesignates operation with a sine wave, or “hard switching” designatingoperation with a rectangular wave. The soft switching power supplies areconsidered to be superior in terms of power conversion efficiency, noiselevel, cost, and so forth. A large screen television typically requiresa switching power supply capable of operation with AC input voltage of100V and maximum load power of 160 W.

Prior art soft switching power supplies are described, for example, inan article entitled “A New Magnetic Flux Control SMPS and the MultiscanDeflection System”, by Masayuki Yasumura et al., IEEE InternationalConference on Consumer Electronics, Jun. 4, 1986. The switching powersupplies described in that article contain four basic components: apower regulating transformer (PRT), a power isolation transformer (PIT),a converter drive transformer (CDT), and a resonant switching element.The characteristics of the PRT may be controlled by a feedback signalbased on the measured output voltage, so as to adjust the switchingfrequency of the switching element, which in turn results in the outputvoltage being modified. In this manner, the output voltage is maintainedin a desired range.

Present day switching power supplies of the self-excited oscillationtype often have the following disadvantage: there is the possibilitythat a high voltage may be applied to the switching transistor, whichrequires the switching transistor to have a high voltage withstandingproperty, e.g., 1800V. Consequently, this property limits the ability toincrease the switching frequency of the device because the powerconversion efficiency is deteriorated by an increase of the power losswhen the switching frequency is increased beyond a certain level.

In other configurations designed to alleviate this problem, universalswitching transistors with lower voltage withstanding properties havebeen used, e.g., 1500V. However, these configurations are of increasedcomplexity and size, requiring typically four large switchingtransistors and isolated signal transmission means such as aphoto-coupler to keep the primary and secondary sides isolated from oneanother.

Accordingly, a need exists for a switching power supply capable of usinga relatively low voltage withstanding transistor(s) and having a simpleconfiguration.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide aswitching power supply that uses a switching element of a desiredvoltage withstanding property, and which achieves a desired AC/DCconversion output.

Another object of the present invention is to provide a switching powersupply with a simple configuration.

In accordance with the invention, there is provided a power supplyincluding a converter transformer having primary and secondary windingsloosely coupled to one another, and an auxiliary winding with apredetermined number of turns wound contiguously from one end of theprimary winding to an end portion of the converter transformer. An inputvoltage smoothing capacitor is connected on one side to an end of theprimary winding of the converter transformer through a forwardlydirected diode. A switching element is connected to the other end of theprimary winding. A control means controls the switching element toperform switching so that an output voltage of the secondary winding hasa predetermined voltage value (or is within a predetermined voltagerange). A second capacitor is coupled across opposite ends of theauxiliary winding via the forwardly directed diode, to boost-up voltageapplied to the converter transformer.

The converter transformer may be comprised of an EE-shaped ferrite corehaving a pair of middle magnetic legs shorter than two pairs of outermagnetic legs thereof such that a gap is formed between the pair ofmiddle magnetic legs. The primary winding is wound around one of themiddle magnetic legs, the secondary winding is wound around the other ofthe middle magnetic legs, so as to provide the loose coupling betweensaid primary and secondary windings.

The control means may be formed as a self-excited oscillation switchingfrequency control means for varying an inductance of a controltransformer with an output voltage of the secondary winding to controlan oscillation frequency. Alternatively, the control means may be formedas a separately excited oscillation switching frequency control meansfor detecting the output voltage of the secondary winding andcontrolling an oscillation frequency of an oscillation circuit with thedetected value.

Advantageously, with the power supply of the present invention, theswitching frequency can be controlled by separately excited oscillation,the voltage to be applied to the primary winding can be boosted up, andthe voltage to be applied to the switching element can be setarbitrarily. Consequently, a desired output can be obtained using aswitching element of a desired voltage withstanding property and adevice having superior characteristics can be used to achieve efficientpower conversion. Further, the number of parts can be reduced to reducethe area of a circuit board thereby to miniaturize the entire apparatus.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description and theappended claims, taken in conjunction with the accompanying drawings inwhich like parts or elements denoted by like reference symbols.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an embodiment of a power supply inaccordance with the invention;

FIG. 2 is a perspective view of an insulating converting transformerthat may be used within embodiments of the invention;

FIG. 3 is a perspective view of an orthogonal control transformer thatmay be used within embodiments of the invention;

FIG. 4 is a diagram illustrating a relationship between a switchingfrequency and a secondary side DC output voltage in a power supply ofthe present invention;

FIG. 5 is a circuit diagram of an alternative embodiment of a powersupply in accordance with the invention;

FIGS. 6A and 6B are circuit diagrams showing alternative output circuitportions of power supplies in accordance with the invention; and

FIG. 7 is a circuit diagram of a related art switching power supply.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, a first embodiment of a power supply in accordancewith the invention is schematically illustrated. Briefly, power supply100 is an AC/DC converter which includes an orthogonal controltransformer (or “power regulating transformer”) PRT having a secondarywinding inductance controlled by a feedback signal from a controlcircuit 3; an insulating converter transformer (or “power isolationtransformer”) PIT; and a transistor Q1 operating as a switching element,such that a self-oscillating feedback loop is formed between elementsQ1, PIT and PRT. Control circuit 3 detects the output voltage of thepower supply across terminals Eo and Eo′, and controls the frequency ofoscillation of the feedback loop in accordance with the detected outputvoltage to thereby maintain the output voltage in a desired range. Animportant aspect of power supply 100 is the provision of diode D1,capacitor C1, and a boost-up winding T3 of transformer PIT, whichtogether enable a switching transistor with lower voltage withstandingvoltage and superior characteristics to be used than would be otherwisepossible, and allows the load power to be increased, as will beexplained below.

A more detailed explanation of power supply 100 will now be presented.The opposite ends of an alternating current power supply A1 areconnected to AC input terminals of a diode bridge circuit Di through aresistor Ri for in-rush current limitation upon connection of powersupply 100 and so forth. A rectifier output terminal of the negativeelectrode side of the diode bridge circuit Di is grounded, and the otherrectifier output terminal of the positive electrode side of the diodebridge circuit Di is grounded through a smoothing capacitor Ci.Consequently, the AC power supply A1 is full-wave rectified by the diodebridge circuit Di, and the rectified voltage is smoothed by capacitorCi. A DC voltage Ei corresponding to an AC input voltage VAC of powersupply A1 is formed on the positive electrode side of capacitor Ci.

Insulating converter transformer PIT functions to transmits power fromthe primary side (with windings T1 and T3) to the secondary side(winding T2) while providing DC isolation between the primary andsecondary sides. The primary winding T1 has N1 turns, and a wireextending from one end of the primary winding T1 to the top end of thetransformer is wound to form a boost-up winding T3 (also referred to asan auxiliary winding) having N3 turns. The primary side can also beconsidered a single, center tapped primary winding (comprising windingsT1 and T3), with the cathode of diode D1 connected to a center tap thatseparates winding T1 and boost-up winding T3.

FIG. 2 depicts a preferable configuration for the insulating convertertransformer PIT. The PIT has an EE-shaped core 105 which includes a pairof E-shaped cores 101 and 102 made of a ferrite material and combinedsuch that magnetic legs thereof oppose each other. Primary winding T1with N1 turns and secondary winding T2 with 2*N2 turns (where N1 is notnecessarily a different number than N2) are wound separately from oneanother on the central magnetic legs of EE-shaped core 105 using a splitbobbin whose winding portion is split between the primary and secondarysides. Boost-up winding T3 (with N3 turns) runs contiguously along thecentral magnetic leg of E-shaped core 101 from the end of primarywinding T1. A gap G is formed between the central magnetic legs of theEE-shaped core 105. The gap G can be created by forming the centralmagnetic legs of the E-shaped cores 101 and 102 shorter than the twoouter magnetic legs. Consequently, a loose coupling, having a lowercoupling coefficient than in a conventional insulating convertertransformer is obtained, and consequently, a saturation condition isless likely to occur. The coupling coefficient k in this instance is,for example, k≈0.85.

Returning to FIG. 1, the positive electrode side of capacitor Ci isconnected to an end of the primary winding T1 of the insulatingconverter transformer PIT through the forwardly directed “boost-up”diode D1. Diode D1 is referred to as a boost-up diode because itfunctions in conjunction with boost-up winding T3 and capacitor C1 toboost the voltage to the primary side of the PIT. An end of “boost-up”capacitor C1 is connected to the positive electrode side of thecapacitor Ci. The other end of the capacitor C1 is connected to theother end of boost-up winding T3 through a controlled winding Ta oforthogonal control transformer PRT. It is to be noted that theorthogonal control transformer PRT further has a controlled winding Tband a control winding Tc, and the inductance values of the controlledwindings Ta and Tb are varied in accordance with a control signalsupplied to the control winding Tc as hereinafter described.

The other end of primary winding T1 is grounded through thecollector-emitter of an npn bipolar junction transistor (BJT) Q1, and aresonance capacitor CR1 is provided in parallel with thecollector-emitter of transistor Q1. A damping diode D11 is providedbetween the base and the emitter of transistor Q1, and the base oftransistor Q1 is connected to the positive electrode side of thecapacitor Ci through a “starting” resistor RS1. The base of transistorQ1 is grounded through a damping resistor R11, a resonance capacitor C11and the controlled winding Tb of the orthogonal control transformer PRT.

The switching converter circuit is formed from the components describedabove. Thus, if the AC power supply A1 is applied to the switchingconverter circuit, then a self-excited oscillation circuit composed ofthe transistor Q1, resistor R11, resonance capacitor C11 and controlledwinding Tb of transformer PRT is started in response to a startingcurrent from the resistor RS1 to start the switching drive of transistorQ1. Then, within a period within which the transistor Q1 is off,resonance current in the form of a sine wave pulse is formed by aresonance circuit which is composed of leakage inductances of thecontrolled winding Ta of the orthogonal control transformer PRT and theboost-up winding T3 and the primary winding T1 of the insulatingconverter transformer PIT, the resonance capacitor CR1 and so forth.

Thereupon, a voltage corresponding to the ratio in turn number betweenthe primary winding T1 of turn number N1 and the boost-up winding T3 ofturn number N3 is generated in the boost-up capacitor C1 describedabove. In other words, a potential VB at the other end of the capacitorC1 is given, where the voltage drop of diode D1 is represented by VF andthe voltage drop between the collector and the emitter of transistor Q1is represented by VCE(SAT), by

VB=[Ei−VF−VCE(SAT)]×[(N 1+N 3)/N 1]≈Ei[1+(N 3/N 1)]

and resonance current corresponding to the potential VB is supplied tothe primary winding T1.

Then, an arbitrary voltage is induced in secondary winding T2 inaccordance with the resonance current which flows through primarywinding T1. The turn number of secondary winding T2 is 2*N2, and acenter tap is provided at a position of the secondary winding T2 atwhich the turn number is equally divided into two. The center tap isgrounded on the secondary side. A capacitor C3 for resonance is providedbetween the opposite ends of secondary winding T2, and a voltageresonance circuit is formed from a leakage inductance of secondarywinding T2 and capacitor C3.

A pair of taps at the opposite ends of secondary winding T2 of theinsulating converter transformer PIT are connected to each other throughforwardly directed diodes D3 and D4, and a junction between them isgrounded on the secondary side through a smoothing capacitor C4. Anoutput terminal 1 is provided at the junction. Consequently, anarbitrary DC output voltage E0 obtained by full-wave rectification of anoutput between the opposite ends of the secondary winding T2 isextracted from output terminal 1.

A pair of intermediate taps are provided at two arbitrary positions ofsecondary winding T2 which are symmetrical with each other with respectto the center tap described above. The intermediate taps are connectedto each other through forwardly directed diodes D5 and D6, and ajunction between them is grounded on the secondary side through asmoothing capacitor C5. Further, another output terminal 2 is providedat the junction. Consequently, a desired DC output voltage E0′ obtainedby full-wave rectification of an output between the arbitraryintermediate taps of the secondary winding T2 is extracted fromsecondary winding T2.

The orthogonal insulating converter transformer PRT functions totransmit a switching output of the switching element Q1 to the secondaryside thereof and to perform constant voltage control of the secondaryside output thereof. Transformer PRT includes, for example, as shown inFIG. 3, a three dimensional core 200 which is formed such that twodouble channel-shaped cores 201 and 202 each having four magnetic legsare joined to each other at the ends of the magnetic legs thereof. Theprimary winding N1 (corresponding to controlled winding Ta of FIG. 1)and a secondary winding N2 (corresponding to controlled winding Tb ofFIG. 1) are wound in the same winding direction around two predeterminedones of the magnetic legs of the three dimensional core 200 and acontrol winding Nc (Tc) is wound around two predetermined ones of themagnetic legs of the three dimensional core 200 such that the windingdirection thereof is orthogonal to the primary winding N1 and thesecondary winding N2. As a result, the orthogonal insulating convertertransformer PRT is formed as a saturable reactor. In this instance, theopposing faces of the opposing legs of the double channel-shaped cores201 and 202 are joined together and have no gap formed therebetween.

Returning to FIG. 1, the DC output voltages Eo and Eo′ are detected bycontrol circuit 3, and a control signal from the control circuit 3 issupplied to the control winding Tc of the orthogonal control transformerPRT. Consequently, the inductance values of the controlled windings Taand Tb are controlled in accordance with the control signal from controlcircuit 3. Then, if the DC output voltage Eo or Eo′ of the outputterminal 1 or 2 varies, (or if the difference voltage Eo−Eo′ varies)then the inductance values of the controlled windings Ta and Tb arevaried, and the conduction angle of the transistor Q1 and theoscillation frequency of the self-excited oscillation circuit describedabove are controlled in a direction to cancel the variation. In thismanner, the DC output voltages Eo and Eo′ to be extracted from theoutput terminals 1 and 2 respectively, are stabilized. (Note that the DCoutput voltage of power supply 100 may be considered the differencebetween Eo and Eo′.)

In power supply 100, the voltage to be applied to switching transistorQ1 can be set arbitrarily in accordance with the ratio between the turnnumber N1 of primary winding T1 and the turn number N3 of boost-upwinding T3. Therefore, if a universal type transistor having a voltagewithstanding property of, for example, 1,500 V is used for transistorQ1, then if the turn number N3 of boost-up winding T3 is selected so asto satisfy 0.5N1 N3 N1, then the voltage to be applied to the transistorQ1 can be controlled always to 1,500 V or less. This allows use of auniversal transistor having superior characteristics.

FIG. 4 further illustrates the principles of operation of power supply100. An exemplary relationship between the switching frequency fs andthe secondary side DC output voltage (i.e., E0 minus E0′) is depicted.(Actually, the shown relationship as depicted by the solid line curvecorresponds more closely to the performance realizable in a power supplydescribed in the above-noted co-pending patent application, which doesnot include the boost up components C1, D1 and T3 of the presentinvention. By using the boost up components, even better performance isrealizable.) In FIG. 4, the abscissa indicates the switching frequency,and the ordinate indicates the level of the secondary side DC outputvoltage Eout=(Eo−Eo′). A resonance curve of dotted lines 75 illustratesa characteristic in a related art power supply circuit shownschematically in FIG. 7. As can be seen from FIG. 4, for example, inorder to make the secondary side DC output voltage Eout a constantvoltage so that it may be Eout=135 V in response to a load variation, itis necessary to control the switching frequency fs within a range of 150KHz from 75 KHz to 225 KHz. However, where the construction of the powersupply circuit of FIG. 7 is used as is, the switching frequency ofswitching element Q1 has a limit approximately at 50 KHz for its voltagewithstanding property. In contrast, with the circuit of FIG. 1, theswitching frequency fs may be controlled within the range of 75 KHz from100 KHz to 175 KHz as can be seen from the solid line resonance curve 78in FIG. 4, and this control range is approximately one half thatobtained with the FIG. 7 apparatus.

In power supply 100, if a rise of the AC input voltage VAC of the ACpower supply A1 or a drop of the load power occurs, then control isperformed (via control circuit 3) so that the switching frequency of thetransistor Q1 may be increased by an action of the orthogonal controltransformer PRT and another control is performed so that the conductionangle of transistor Q1 may be reduced. Consequently, a voltage resonancepulse voltage Vcp to be applied to the transistor Q1 and the resonancecapacitor CR1 exhibits the highest peak value when the switchingfrequency is lowest while the AC voltage is lowest and the load ishighest.

The voltage resonance pulse voltage Vcp is given by

Vcp=VB[1+(π/2)×(Ton/Toff)]

Therefore, if the turn number N3 of boost-up winding T3 and the turnnumber N2 of secondary winding T2 satisfies N3=N2, for example, when theAC input voltage VAC is 80 V, the potential VB is 220 V, the on time Tonof the transistor Q1 is 3×TOFF and the voltage resonance pulse voltageVCP is 1,250 V. Further, when VAC=120 V, VB=330 V, and Ton=Toff.Consequently, VCP=860 V.

In power supply 100, the apparent AC input voltage is doubled and anincrease of the maximum load power is achieved as compared to the casethat does not include the boost-up circuitry, namely capacitor C1, diodeD1, winding T3 (and resistor Rs1). Therefore, the power supply iseffective also where, for example, the maximum load power is 160 W ormore. Further, in this case, the voltage withstanding property requiredfor the transistor Q1 can be made 1,500 V or less to allow use of auniversal device for transistor Q1, and efficient power conversion canbe achieved by use of a device having superior characteristics. Forexample, an efficiency of 90% or more can be maintained despitevariation of the AC input voltage.

Further, where a DC output voltage is obtained by voltage resonance bythe circuit of the secondary winding T2 of the insulating convertertransformer PIT, if the middle magnetic legs of the EE-shaped ferritecore of the insulating converter transformer PIT are formed shorter thanthe other outer side magnetic legs to form a gap therebetween so as toprovide a loose coupling (for example, the coupling coefficient≈0.85)between the primary Winding and the secondary winding, then theinsulating converter transformer PIT becomes less likely to suffer froma saturation condition and the conversion is performed with enhancedstability.

Accordingly, in the above-described power supply, by: 1) applying thepositive pole of an input smoothing capacitor to one end of the primarywinding of the converter transformer through a diode; 2) winding apredetermined winding contiguously from the one end of the primarywinding; and 3) then providing the capacitor between the opposite endsof the thus wound winding to boost up the voltage to be applied to theprimary winding, the number of turns of the boost-up winding can beadjusted to arbitrarily set the voltage to be applied to the switchingelement. Therefore, a desired voltage output can be obtained using aswitching element of a desired voltage withstanding property and adevice of a superior characteristic can be used to achieve efficientpower conversion. Further, the number of parts can be reduced to reducethe area of a circuit board to miniaturize the entire apparatus.

Advantageously, power supply 100 eliminates the above-noted problems ofprior art switching power supplies. That is, a conventional power supplyapparatus of the self-excited oscillation type is disadvantageous inthat there is the possibility that a high voltage may be applied to aswitching transistor and, since a device having a high voltagewithstanding property is required, a limitation to increase in switchingfrequency is provided and the power conversion efficiency isdeteriorated by an increase of the power loss. Moreover, an apparatus,for example, of the separately excited oscillation type isdisadvantageous in that, although the problem of the withstandingvoltage of a switching element is eliminated, a greater number of partsare required and a greater area is required for a printed circuit boardor the like on which such circuit parts are to be mounted, resulting ina large size of the entire apparatus. On the other hand, with the powersupply apparatus of the present invention, these problems can be readilyeliminated.

FIG. 5 illustrates an alternative embodiment of a power supply, 300, inaccordance with the present invention. Power supply 300 is differs fromthe power supply apparatus 100 of FIG. 1 in that a pair of transistorsQm1 and Qm2 connected in a Darlington connection are used as theswitching element in place of the transistor Q1 described hereinabove.It is to be noted that overlapping description of common components isomitted herein to avoid redundancy.

In power supply 300, the lower end of primary winding T1 of theinsulating converter transformer PIT described hereinabove is groundedthrough the collector-emitter of the transistor Qm2 connected in aDarlington connection, and a resonance capacitor CR1 is connected inparallel to the collector-emitter of the transistor Qm2 together with adamping diode Dm1. The collector of transistor Qm1 is connected to thecollector of transistor Qm2, and the emitter of the transistor Qm1 isconnected to the base of transistor Qm2. Also, resistors Rm1 and Rm2 areconnected to the bases and the emitters of the transistors Qm1 and Qm2,respectively, and the base of the transistor Qm1 is connected to theemitter of the transistor Qm2 through a damping diode Dm2.

A control signal from control circuit 3 is supplied to an oscillationand drive circuit 4, and a driving signal from the oscillation and drivecircuit 4 is supplied to the base of the transistor Qm1. It is to benoted that, in this instance, isolated signal transmission means such asa photo-coupler is required between the control circuit 3 and theoscillation and drive circuit 4. Further, a voltage on the positiveelectrode side of the smoothing capacitor Ci described above is suppliedto the oscillation and drive circuit 4 through a starting resistor Rms.Consequently, the oscillation and drive circuit 4 forms a drive signalmodulated in frequency and pulse width, for example, in accordance withthe control signal from the control circuit 3 described above. The drivesignal is supplied to the base of the transistor Qm1 of the transistorsQm1 and Qm2 connected in a Darlington connection.

Accordingly, also in the power supply apparatus of FIG. 5, by applyingthe positive pole of an input smoothing capacitor to one end of theprimary winding of the converter transformer through a diode and windinga predetermined winding contiguously from the one end of the primarywinding and then providing the capacitor between the opposite ends ofthe thus wound winding to boost up the voltage to be applied to theprimary winding, the number of turns of the boost-up winding can beadjusted to arbitrarily set the voltage to be applied to the switchingelement, and a desired output can be obtained using a switching elementof a desired voltage withstanding property and a device of a superiorcharacteristic can be used to achieve efficient power conversion. Also,the load power can be increased by virtue of the boosted-up voltageapplied to transformer PIT. Further, the number of parts can be reducedto reduce the area of a circuit board to miniaturize the entireapparatus.

It is to be noted that, for the switching element used in the powersupply apparatus described hereinabove with reference to FIGS. 1 and 5,as an alternative to using a single npn bipolar transistor for switchingor transistors connected in a Darlington connection (Darlington BJTs) asdescribed above, a MOSFET(Metal Oxide Semiconductor Field EffectTransistor), an IGBT (insulated gate bipolar transistor), a SIT(electrostatic induction thyristor) or the like can be used.

Further, in the power supply apparatus of FIG. 5, extraction of the dcoutput voltage from the insulating converter transformer PIT isperformed by a voltage multiplying rectification system wherein acurrent resonance circuit formed from a series connection of a secondarywinding and a resonance capacitor is used. In particular, in this case,diodes D21 and D22 connected in series in a forward direction from thegrounded end of the secondary side toward an output terminal 21 areprovided. An end of the secondary winding Tm2 of the insulatingconverter transformer PIT is grounded on the secondary side, and theother end of secondary winding Tm2 is connected to a junction betweenthe diodes D21 and D22 through a capacitor C21. Further, a capacitor C22is connected in parallel to the series circuit of the diodes D21 andD22.

Further, the center tap of a secondary winding Tm3 is grounded on thesecondary side, and taps at the opposite ends of the secondary windingTm3 are connected to each other through forwardly directed diodes D23and D24. A junction between them is grounded on the secondary sidethrough a capacitor C23 for smoothing, and another output terminal 22 isled out from the junction.

Consequently, a DC output voltage of a multiplied voltage can beobtained from the current resonance circuit of the secondary windingTm2. Further, an output between the arbitrary intermediate taps of thesecondary winding Tm3 is full-wave rectified and a desired dc outputvoltage is extracted. Further, in the power supply apparatus, a desiredoutput can be obtained using a switching element of a desired voltagewithstanding property and a device with superior characteristics can beused to achieve efficient power conversion. Furthermore, the number ofparts can be reduced to reduce the area of a circuit board tominiaturize the entire apparatus.

FIG. 6A shows a circuit wherein extraction of a DC output voltage froman insulating converter transformer PIT is performed by full-waverectification using bridge rectification with a voltage resonancecircuit wherein a resonance capacitor is connected in parallel to thesecondary winding. (Note that the circuits of FIG. 6A or 6B can be usedto replace the output circuitry in the power supplies 100 or 300 ofFIGS. 1 and 5, respectively.) In the circuit shown, a resonancecapacitor Cm3 is connected between the opposite ends of a secondarywinding Tm4 of the insulating converter transformer PIT and connected toAC input terminals of a diode bridge circuit Dm. A rectifier outputterminal of the negative electrode side of the diode bridge circuit Dmis grounded on the secondary side, and the other rectifier outputterminal of the positive electrode side is grounded on the secondaryside through a capacitor Cm4 for smoothing. Further, an output terminal23 is provided at the latter rectifier output terminal of the positiveelectrode side of the diode bridge circuit Dm.

Consequently, a DC output voltage can be obtained by full-waverectification using bridge rectification with a voltage resonancecircuit formed for the secondary winding Tm4. Further, also in thecircuit shown in FIG. 6A, a desired output can be obtained using aswitching element of a desired voltage withstanding property and adevice of a superior characteristic can be used to achieve efficientpower conversion. Furthermore, the number of parts can be reduced toreduce the area of a circuit board to miniaturize the entire apparatus.

FIG. 6B shows another circuit wherein extraction of a DC output voltagefrom the insulating converter transformer PIT is performed by aquadruple voltage rectification system wherein a current resonancecircuit composed of a series connection of two resonance capacitors tothe secondary winding is used. In particular, in the circuit shown inFIG. 6B, diodes D31, D32, D33 and D34 connected in series in a forwarddirection from the grounded end of the secondary side toward the outputterminal 31 are provided. An end of a secondary winding Tm5 of theinsulating converter transformer PIT is connected to a junction of thediodes D32 and D33, and the other end of the secondary winding Tm5 isconnected to a junction of the diodes D31 and D32 and a junction of thediodes D33 and D34 through capacitors C31 and C32, respectively.

Further, a capacitor C33 is connected in parallel to the series circuitof the diodes D31 and D32, and another capacitor C34 is connected inparallel to the series circuit of the diodes D33 and D34. Consequently,a DC output voltage of a quadruple voltage can be obtained from thecurrent resonance circuit of the secondary winding Tm5. Also in thecircuit shown in FIG. 6B, a desired output can be obtained using aswitching element of a desired voltage withstanding property and aswitching device (transistor) with superior characteristics can be usedto achieve efficient power conversion. Furthermore, the number of partscan be reduced to reduce the area of a circuit board to miniaturize theentire apparatus.

While the present invention has been described above in reference topreferred embodiments thereof, it is understood that these embodimentsare merely exemplary and that one skilled in the art can make manychanges to the disclosed embodiments without departing from the spiritand scope of the invention as defined by the appended claims.

Industrial Applicability

As described above, a power supply according to the present inventionincludes a converter transformer having primary and secondary windingsloosely coupled to one another, and an auxiliary winding with apredetermined number of turns wound contiguously from one end of theprimary winding to an end portion of the converter transformer. An inputvoltage smoothing capacitor is connected on one side to an end of theprimary winding of the converter transformer through a forwardlydirected diode. A switching element is connected to the other end of theprimary winding. A control circuit controls the switching element toperform switching so that an output voltage of the secondary winding hasa predetermined voltage value (or is within a predetermined voltagerange). A second capacitor is coupled across opposite ends of theauxiliary winding via the forwardly directed diode, to boost-up voltageapplied to the converter transformer. Thus, with the power supplyaccording to the present invention, a desired conversion output can beobtained using a switching element of a desired voltage withstandingproperty.

What is claimed is:
 1. A power supply comprising: a convertertransformer having primary and secondary windings loosely coupled to oneanother, and an auxiliary winding with a predetermined number of turnswound contiguously from one end of said primary winding to an endportion of said converter transformer; an input voltage smoothingcapacitor having one side thereof connected to an end of said primarywinding of said converter transformer through a forwardly directeddiode; a switching element connected to the other end of said primarywinding of said converter transformer; control means for controllingsaid switching element to perform switching so that an output voltage ofsaid secondary winding has a predetermined voltage value; and a secondcapacitor coupled across opposite ends of said auxiliary winding viasaid forwardly directed diode, to boost-up voltage applied to saidconverter transformer.
 2. The power supply of claim 1, wherein saidconverter transformer comprises: an EE-shaped ferrite core having a pairof middle magnetic legs shorter than two pairs of outer magnetic legsthereof such that a gap is formed between said pair of middle magneticlegs, said primary winding being wound around one of said middlemagnetic legs, said secondary winding being wound around the other ofsaid middle magnetic legs, so as to provide said loose coupling betweensaid primary and secondary windings.
 3. The power supply of claim 1,further comprising a resonance capacitor connected in parallel acrosssaid secondary winding of said converter transformer to form a voltageresonance circuit, wherein a DC output voltage is obtained by full-waverectification using a center tap of said secondary winding.
 4. The powersupply of claim 1, further comprising a resonance capacitor connected inseries to said secondary winding to form a current resonance circuit,wherein a DC output voltage is obtained by a voltage multiplyingrectification means.
 5. The power supply of claim 1, wherein a resonancecapacitor is connected in parallel to said secondary winding of saidconverter transformer to form a voltage resonance circuit, and a DCoutput voltage is obtained by full-wave rectification using bridgerectification.
 6. The power supply of claim 1, wherein a pair ofresonance capacitors are connected in series to said secondary windingof said converter transformer to form a current resonance circuit, and aDC output voltage is obtained by a quadruple voltage rectificationmeans.
 7. The power supply of claim 1, wherein said control means isformed as a self-excited oscillation switching frequency control meansfor varying an inductance of a control transformer with an outputvoltage of said secondary winding to control an oscillation frequency.8. The power supply according to claim 1, wherein said control means isformed as separately excited oscillation switching frequency controlmeans for detecting the output voltage of said secondary winding andcontrolling an oscillation frequency of an oscillation circuit with thedetected value.